In fact, by 2006, overdoses of opioid analgesics alone (a class of pain relievers that includes morphine and methadone) were already causing more deaths than overdoses of cocaine and heroin combined.

"Teens and others have different attitudes in using these drugs," often presuming the prescription substances are safer and less addictive than illegal drugs such as cocaine or heroin, says Jeffrey Coben, a professor of emergency and community medicine at the West Virginia University School of Medicine in Morgantown and lead author of the new study. "I think that's a false assumption. Aside from the fact they can be taken orally rather than injected…[many prescription drugs] really are every bit as powerful, addictive and dangerous as heroin," he notes, adding that, "when you combine them with other sedatives, that mix can become particularly lethal."

Using data collected by the Nationwide Inpatient Sample, which gathers hospital patient information for about 8 million people every year, Coben and his colleagues were able to assess what drugs were implicated in the majority of poisonings—and in many cases whether the poisonings were intentional or not. The team selected opioids, sedatives and tranquilizers as the focus of the analysis because these substances are "contributing the majority of prescription drug overdose deaths," Coben says. These categories of prescription drugs can kill and injure people by suppressing breathing, depriving the body of oxygen.

For prescription opioids, sedatives and tranquilizers—commonly prescribed for pain management—the number of hospitalizations for poisonings increased 65 percent between 1999 and 2006 (the first and last years, respectively, for which data were comparable and collected). The number of hospitalizations for all poisonings, including illegal drugs, other prescription medications and miscellaneous substances, increased during this time period as well, but that jump (33 percent) was about half the rate of those for the prescription pain drugs.

Unintentional poisonings from these drugs climbed 37 percent during the seven-year period, the researchers found. Intentional overdoses, in which people meant to inflict self-harm or death, jumped 130 percent (a far cry more than the 53 percent increase of intentional poisoning from other substances in the same time period). Intent was not listed in all cases and can be subject to reporting error. The results are detailed online April 6 in the American Journal of Preventive Medicine.

No accident
Poisonings, from prescription drugs and other substances, are classified in medical records as injurious or accidental deaths. But regardless of whether the incidents are listed as unintentional or intentional, they are rarely true mistakes, noted Leonard Paulozzi, a medical epidemiologist with the U.S. Centers for Disease Control and Prevention, in congressional testimony in 2007. "Most unintentional drug poisoning deaths are not 'accidents' caused by toddlers or the elderly taking too much medication," he noted. "These deaths are largely due to the misuse and abuse of prescription drugs."

Accidents overall were the fifth most common cause of death in the U.S. as of 2005 (accounting for 117,809 deaths—4.8 percent—that year), according to the National Vital Statistics Report [pdf]. Of injury deaths, poisoning is the second most common cause of death in the U.S., having doubled between 1985 and 2004, according to a 2007 Department of Health and Human Services analysis [pdf]. Among people 35 to 54 years old, poisoning is the most common accidental death—even more so than auto-related deaths.

Many experts think that the sheer prevalence of many of these drugs recently has contributed to the drastic increase in poisonings. Although growing illegal markets and distribution of these drugs might be a driving factor in their increasingly large role in poisonings and deaths, perfectly legal prescriptions are probably playing a role as well, Coben says.

"I think the whole issue of the availability of these drugs and whether they're being over-prescribed" should be investigated, says Susan Baker, a professor at Johns Hopkins Center for Injury Research and Policy, who was not involved in the new study but coauthored a 2009 report in the same journal about recent trends in injury mortality.

Many people do rely on pharmacological treatment for withdrawal, anxiety or chronic pain, but when communities have access to an overabundance of these medications, abuse appears to become more likely. If doctors prescribe too much medication or too many refills, excess drugs "are going to be sitting in people's medicine cabinets for someone else to take advantage of," Baker explains. For example, methadone poisonings were four times as frequent in 2006 as they were in 1999, a time period during which retail sales grew more than 1,000 percent, Coben and his team found.

Antidote unknown
Although the new report details the stark increase in the reported poisoning data, the true number of deaths and hospitalizations in which prescription drugs have played a role might be even higher, the researchers pointed out. The new analysis assessed cases only in which prescription drug overdose was listed as the primary diagnosis. Some prescription drug–related hospitalizations might be classified under other primary categories, and those who abuse the drugs were not always labeled as having been poisoned. Additionally, the researchers explained, many common terms such as overdose, misuse and abuse are not well standardized in hospitals.

"I don't have any sense that it's getting any better," Coben says. With drug companies reporting strong overall sales (including a 5.1 percent increase in U.S. sales in 2009 to $300.3 billion for 3.9 billion individual retail prescriptions), in fact, the problem might be getting worse.

The researchers noted that the details surrounding these hundreds of thousands of overdoses are unknown. The medical data used for the analysis did not include full toxicology reports that would reveal drug-drug interactions. And although the researchers found that the majority of the people hospitalized for poisoning with these prescription drugs were women, they did not have enough other demographic data to propose possible reasons for the overdose increases.

"What we really need is something other than the coded data," Baker says. She notes that researchers need to know more about the circumstances in which people are overdosing before effective prevention measures can be put into place.

"There's a need to have informational interviews with people who have had overdoses and survived them," Coben says. He hopes that future research will "raise some opportunities for interventions with these people."